Add scheduled tasks feature and delayed entity signals

[andystaples]

Summary

Adds a scheduled tasks feature (durabletask.scheduled) to the Python SDK,
bringing it to parity with the Microsoft.DurableTask.ScheduledTasks package in
durabletask-dotnet. A schedule periodically starts a target orchestration based
on a configurable interval and time window. The feature is built entirely on top
of durable entities plus a small helper orchestrator — no new sidecar/protocol
requirements.

To make schedules work, this PR also adds delayed (scheduled) entity signals
across the entity, orchestration-context, and client signal_entity APIs, and
teaches the in-memory test backend to honor them.

What's new

durabletask.scheduled package

• configure_scheduled_tasks(worker) — registers the schedule entity and
  operation orchestrator on a worker (parity with .NET's UseScheduledTasks).
• ScheduledTaskClient — create_schedule, get_schedule, get_schedule_client,
  list_schedules.
• ScheduleClient — per-schedule create, describe, update, pause,
  resume, delete.
• Public DTOs: ScheduleCreationOptions, ScheduleUpdateOptions,
  ScheduleDescription, ScheduleQuery, ScheduleStatus.
• Errors: ScheduleNotFoundError, ScheduleInvalidTransitionError,
  ScheduleClientValidationError.

Schedules support interval (≥ 1s), start_at, end_at, and
start_immediately_if_late, with a state machine (Uninitialized → Active
⇄ Paused) and an execution-token mechanism that cancels stale timer signals,
mirroring the .NET design.

Delayed entity signals (prerequisite)

• Added an optional signal_time parameter to:
  • EntityContext.signal_entity / DurableEntity.signal_entity
  • OrchestrationContext.signal_entity
  • client signal_entity (sync and async)
• These map to the existing protobuf scheduledTime field. The public parameter
  name signal_time matches the .NET public API (SignalEntityOptions.SignalTime).
• The in-memory backend now defers entity operations whose scheduledTime is in
  the future (via a background timer thread), on both the client-signal and
  orchestration-emitted-signal paths.

Tests

• Unit: schedule state machine, transition rules, config validation/diff,
  and state/options serialization round-trips.
• In-memory E2E: create/describe/run, pause/resume, update, list with
  prefix/status filters, delete; plus delayed-signal deferral via client and
  orchestrator.
• Live DTS E2E (pytest.mark.dts): scheduled-task lifecycle and delayed
  signals, verified against the DTS emulator.

Docs & examples

• New examples/scheduled_tasks.py.
• CHANGELOG.md entries under Unreleased.

Notes

• Placed in the core durabletask package (not a separate distribution as in
  .NET) for simpler installation and testing.
• Field names are Pythonic snake_case; class names stay aligned with .NET.
• The schedule entity name is lowercased to schedule, so list filtering uses
  the @schedule@<prefix> instance-ID prefix.

[Copilot]

Pull request overview

This PR introduces a new scheduled tasks capability (durabletask.scheduled) to the core Durable Task Python SDK, implemented on top of durable entities plus a helper orchestrator. As a prerequisite, it also adds delayed (scheduled) entity signals end-to-end (entity APIs, orchestration context, client APIs, and the in-memory test backend) to support time-based schedule re-arming.

Changes:

• Added the durabletask.scheduled package (schedule entity, client surface, models/DTOs, transition rules, registration helper, and error types).
• Added a signal_time parameter across entity/orchestration/client signal_entity APIs and mapped it to the protobuf scheduledTime field.
• Updated the in-memory backend to defer entity operations scheduled in the future; added unit/E2E coverage plus a runnable example and changelog entry.

Reviewed changes

Copilot reviewed 28 out of 28 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
tests/durabletask/scheduled/test_transitions.py	Unit tests for schedule status transition rules.
tests/durabletask/scheduled/test_scheduled_e2e.py	In-memory E2E coverage for schedule lifecycle and runs.
tests/durabletask/scheduled/test_schedule_entity.py	Unit tests for schedule entity behavior/actions via executor harness.
tests/durabletask/scheduled/test_models.py	Unit tests for scheduled-task models, validation, and serialization.
tests/durabletask/scheduled/init.py	Test package init.
tests/durabletask/entities/test_delayed_signals_e2e.py	In-memory E2E coverage for delayed entity signals (client + orchestrator).
tests/durabletask-azuremanaged/scheduled/test_dts_scheduled_e2e.py	Live DTS E2E coverage for scheduled tasks.
tests/durabletask-azuremanaged/scheduled/init.py	Azuremanaged test package init.
tests/durabletask-azuremanaged/entities/test_dts_delayed_signals_e2e.py	Live DTS E2E coverage for delayed entity signals.
examples/scheduled_tasks.py	End-to-end sample showing schedule creation and management.
durabletask/worker.py	Extends orchestration-context signal_entity plumbing to accept signal_time.
durabletask/testing/in_memory_backend.py	Implements delayed entity operation delivery in the in-memory backend.
durabletask/task.py	Updates orchestration-context API contract/docs to include signal_time.
durabletask/scheduled/transitions.py	Defines valid schedule state transitions keyed by operation.
durabletask/scheduled/schedule_status.py	Introduces ScheduleStatus enum.
durabletask/scheduled/schedule_entity.py	Implements the schedule entity state machine and re-arming via delayed signals.
durabletask/scheduled/registration.py	Adds configure_scheduled_tasks(worker) registration helper.
durabletask/scheduled/orchestrator.py	Adds helper orchestrator for “awaitable” schedule entity operations.
durabletask/scheduled/models.py	Adds public DTOs/options plus internal config/state serialization.
durabletask/scheduled/exceptions.py	Adds scheduled-task exception types.
durabletask/scheduled/client.py	Adds ScheduledTaskClient and per-schedule ScheduleClient.
durabletask/scheduled/init.py	Exposes the scheduled tasks public surface.
durabletask/internal/helpers.py	Adds scheduled time support to entity-signal orchestrator actions.
durabletask/internal/client_helpers.py	Adds scheduled time support to client SignalEntityRequest.
durabletask/entities/entity_context.py	Adds signal_time to entity-side signal_entity API and action emission.
durabletask/entities/durable_entity.py	Adds signal_time to DurableEntity.signal_entity facade.
durabletask/client.py	Adds signal_time to sync + async client signal_entity APIs.
CHANGELOG.md	Documents the new scheduled tasks feature and delayed entity signals.

[berndverst]

Review summary

Thanks @andystaples — this is a solid, well-tested feature that mirrors the .NET ScheduledTasks design closely: the Schedule entity state machine, the execution-token mechanism that cancels stale timer signals, and the per-operation orchestrator so client writes can await entity completion. Lint is clean and the scheduled + delayed-signal unit/in-memory-E2E tests pass locally (48 passed).

The recent commits are good improvements: making ScheduleState.from_json converter-free means state round-trips under any code path (not just the worker's threaded converter), and signal_time -> scheduledTime is now correctly threaded through both the client request builder and the orchestrator-action builder (these previously hardcoded scheduledTime=None).

A few things I'd like addressed or explicitly deferred (details inline):

Recommend before merge

• Naive/aware datetime crash in ScheduleClient._matches_filter: comparing an aware schedule_created_at against a possibly-naive created_from/created_to raises TypeError. Cheap to fix + test.
• Missing WORKER_CAPABILITY_SCHEDULED_TASKS advertisement: .NET advertises this capability; Python defines it in the protobuf but never sends it. Either wire it through configure_scheduled_tasks / worker.py, or confirm the backend doesn't gate on it.

Reasonable follow-ups (these match current .NET behavior)

• Typed schedule exceptions flattened to RuntimeError across the orchestration boundary.
• list_schedules applies status/created filters client-side, so pages may be underfilled.
• Exclusive created_from/created_to bounds (consider inclusive).
• Cross-SDK entity-name casing (@schedule@ vs @Schedule@).
• No async client surface (the SDK already has an async client; .NET offers async).

Overall: mergeable with minor fixes — I'd gate on the datetime fix and a decision on the capability flag; the rest can be tracked as follow-ups. Nice work.